Optics, particularly reflective optics, is an important part of elements employed in Extreme Ultra-Violet (EUV) lithography. These elements are used with extreme ultraviolet radiation to illuminate, project, and reduce pattern images that are utilized to form integrated circuit patterns. The use of extreme ultraviolet radiation is beneficial in that smaller integrated circuit features can be achieved; however, the manipulation of the radiation in this wavelength range raises challenges.
In these and similar applications, low thermal expansion glass, such as silica-titania glass, is currently being used for making projection optics. In contrast to other materials, low thermal expansion glass provides improved polishability, improved coefficient of thermal expansion (CTE) control, and improved dimensional stability. However, as the development of these and similar applications proceeds, the demand for improved material characteristics grows.
Ultra-low expansion (ULE) glass has conventionally been made using laydown processes, where the glass is formed in layer deposits. One limitation of ULE glass made in accordance with a laydown process is that the resulting glass contains striae. Striae are compositional inhomogeneities which adversely affect optical transmission in elements made from the glass. Striae result in alternating thin layers of different CTE and therefore alternating planes of compression and tension between the layers. Striae in ULE glass are evident in the direction parallel with the top and bottom of the glass. For example, in silica-titania glass, the striae may include variations in titania composition of generally more than about 0.10 wt. % as compared to the local average titania level.
In some cases, striae have been found to impact surface finish at an angstrom root mean square (rms) level in reflective optic elements, which can adversely affect the polishability of the glass. Polishing glass having striae results in unequal material removal and unacceptable surface roughness which can present problems for stringent applications such as for EUV lithography elements. For example, it may create a mid-frequency surface structure that may cause image degradation in mirrors used in the projection systems for EUV lithography.
Additionally, in contrast with other glass forming methods such as batch melting, laydown processes involve multiple time consuming steps and a greater amount of material. As such, laydown processes are also more costly than other glass forming methods. However, because of the high viscosity and high slope of CTE versus temperature of the glass, it has previously been unfeasible to form ULE glass using conventional batch melting techniques.